Let's say I started serious weightlifting (olympic) a year ago and within a few months ago started smoking regularly. Once (yes only once) a day, after all other responsibilities are done and with a vaporizer (no tar and other negative stuff from combustion).

Does the THC slow down your reaction/speed of the CNS? Can it negatively affect work capacity?

I've met a fair share of guys who smoke once daily and train hard and heavy, and it never seemed to affect them negatively. More than anything, they claimed it helped relax them for better recovery and stimulated their appetite so much more that it was finally possible to cram down all the food they needed to eat

If you're vaporizing, you'll probably be even better off without the tar and other crap to inhale. If you don't feel worse for the wear and don't find that your progress is stalling sooner than it should, I don't think you have much to worry about. Certainly less damage than those people who go out and drink a half dozen pints after training!

"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous